(1) Field of the Invention
The present invention relates to a method of driving a non-volatile memory.
(2) Description of the Related Art
In recent years, cellular phones and personal digital assistants (PDA) are increasingly required to handle a large amount of image information, and thus high-speed, low-power-consuming and small-sized non-volatile memories with a high capacity are desired. Especially, so-called phase change memory devices, which use materials whose resistance values are characteristically varied as a whole depending on their crystal states, are recently drawing attention as ultra-highly integrated memory devices which enable non-volatile operation.
This device has a relatively simple structure in which phase change material comprising a plurality of chalcogen elements is sandwiched between two electrode materials. An electric current is applied between two electrodes to give Joule heat to the phase change material, whereby the crystal state of the phase change material is changed between an amorphous phase and a crystalline phase. Thus, recording of data is accomplished. A GeSbTe-based phase change material, for example, usually contains more than one crystal phase mixed together. Therefore, it is theoretically possible to change the resistance value between the two electrodes in an analog manner. For this reason, such phase change materials are expected to be applied to not only digital memories but also analog memories that can record multi-valued data.
Since the crystal state in the memory active region of a phase change material is very stable at room temperature, it is considered that it can retain data for far more than ten years. For example, U.S. Pat. No. 5,296,716 (hereinafter referred to as patent document 1) by Ovshinsky is a reference that shows the technology level of a phase change memory.
Moreover, the constitution of a phase change memory cell using field-effect transistor as switching elements is disclosed in U.S. Pat. No. 6,314,014 (hereinafter referred to as patent document 2) by Lowrey et al.
FIG. 12 illustrates a prior art phase change memory cell using a field-effect transistor, wherein (a), (b) and (c) are the circuit diagram of the phase change memory cell, a cross sectional view of a variable resistor element using the phase change material, and a figure showing the current voltage characteristics of the variable resistor element using the phase change material, respectively. The circuit diagram (a) is similar to that disclosed in the aforementioned patent document 2. This phase change memory cell comprises a field-effect transistor (hereinafter referred to as MOS) 90, a variable resistor element 91 which has a memory function and consists of a phase change material, a bit line BL for data input/output, a word line WL which is connected to a gate electrode and controls data input/output by turning on/off the MOS 90 and a current or voltage supply section VA. The variable resistor element 91 is formed, for example, as shown in (b). Specifically, the variable resistor element 91 comprises an upper electrode 100, a phase change material film 101 such as GeSbTe (germanium, antimony, tellurium), an interlayer insulation film 103 such as a silicone oxide film, a metal plug 104 which operates as a heat generator and a lower electrode 105. A phase change region 102, which contacts the electrode plug 104, within the phase change material film 101, changes its crystal state as described later.
As shown in FIG. 12(c) with dotted lines, when a voltage is applied to the variable resistor element which is highly resistive (amorphous) in the initial state, almost no current flows until a threshold voltage Vth is attained and therefore little heat is generated. The high resistance state (hereinafter also referred to as a reset state) is thus maintained. When the applied voltage exceeds the threshold voltage Vth, part of the phase change material film 101 (phase change region 102 of FIG. 12(b)) is crystallized due to the Joule heat generated by current. This changes the variable resistor element into a low resistance state (hereinafter also referred to as a set state). In this manner, as described above, a memory function can be achieved by associating the resistance values of the variable resistor element using the phase change material in the set state and reset state with, for example, data 1 and 0, respectively. To return the phase change material which has been changed into the low resistance state to the high resistance state (reset state), the variable resistor element may be rapidly cooled after a current higher than a predetermined threshold current value Ith is applied to the variable resistor element.
As shown in FIG. 12(c), the current region I/Ith>1 that can change the variable resistor element into the high resistance state is referred to as a reset current region, and the current region I/Ith=0.6–1 that can change the element into the low resistance state is referred to as a set current region. Reading of the present resistance value of the variable resistor element needs to be done in a low current region I/Ith<0.6 (applied voltage: about 0.45 V or lower) to avoid read disturbance are (change in a resistance value due to a read-out operation). For example, in the memory cell having the constitution shown in FIG. 12(a), it is necessary to set the voltage applied to the voltage supply section VA to 0.45 V or lower when reading a resistance value.
To change the phase change memory into the reset state, however, a high current of 1 mA or higher needs to be applied to each variable resistor element even in a minute element using a 0.18 μm design rule. For this reason, when a MOS is used as an switch element, there have been the problems of an increased occupied area due to an increased channel width and increased power consumption resulting from the necessity to increase the voltage applied to the gate. In addition, at the time of reading a resistance value, as described above, a sufficient voltage cannot be applied between the source and drain of the MOS, i.e., the switching element, to avoid read disturbance. Accordingly, there is the problem that high-speed read operation is difficult, and therefore high-performance switching elements that can provide high driving performance even during low-voltage operation have been required.